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The camera in your phone, the images from the Hubble Space Telescope and many medical scopes rest on one idea: turn light into electric charge in a grid of tiny silicon cells, then read the charges out in order. Two engineers at Bell Labs sketched the first version in 1969. This post covers who they were, how the idea came about, worked numbers on photons and pixels, and what a student can learn from them.
Who were Willard Boyle and George Smith?
Boyle was born on 19 August 1924 in Amherst, Nova Scotia, and died on 7 May 2011, according to his Nobel facts page. In his Nobel autobiography he writes that his mother home-schooled him until he was fourteen, that he flew Spitfires from aircraft carriers in the Second World War, and that he finished a PhD at McGill in 1950 and joined Bell Labs in 1953.
Smith was born on 10 May 1930 in White Plains, New York, and died on 28 May 2025, according to his facts page. His autobiography describes four years as a US Navy weatherman, a PhD from Chicago in 1959 and a move straight to Bell Labs, into a department headed by Boyle.
They shared half of the 2009 Nobel Prize in Physics "for the invention of an imaging semiconductor circuit – the CCD sensor". The other half went to Charles Kao for optical fibres.
How did a memory idea become a camera?
The Nobel Committee's popular information says that one day in September 1969 the two men outlined the CCD on a blackboard in Boyle's office. They were not thinking about photographs. Their boss had encouraged them to compete with another Bell Labs memory technology, and they wanted a better electronic memory. Once the design was finished, technicians built the first prototype within a week. They published it in 1970 as Charge Coupled Semiconductor Devices in the Bell System Technical Journal.
As a memory, the CCD is forgotten. As an image sensor it spread fast: the popular information records a 100 × 100 pixel sensor in 1972 and the first images of the Moon with a digital sensor in 1974.
How does a CCD turn light into a picture?
The press release explains that the CCD uses the photoelectric effect. Light falling on a silicon cell frees electrons, which collect in that cell like water in a well; more light gives more electrons. A voltage pattern then moves the charges along, row by row, to a single output. The popular information gives the example of a 10 × 10 array becoming a chain of 100 readings.
Worked example: pixels, photons and electrons
Megapixels. A sensor of 1280 × 1024 pixels has 1,310,720 pixels, about 1.3 megapixels, as the popular information states.
Energy of one photon. For green light, λ = 550 nm: E = hc/λ = (6.63 × 10⁻³⁴ × 3.00 × 10⁸) / (550 × 10⁻⁹) = 3.61 × 10⁻¹⁹ J. Divide by 1.602 × 10⁻¹⁹ J/eV: 2.25 eV.
Can it free an electron? The NCERT semiconductor chapter gives silicon's energy gap as about 1.1 eV. Since 2.25 eV > 1.1 eV, a green photon can lift an electron across the gap.
Longest useful wavelength. λ_max = hc/E_g = 1240 eV nm / 1.1 eV ≈ 1130 nm, in the near infrared.
Counting electrons (sample numbers). Suppose one pixel receives 1.0 × 10⁻¹⁶ J of 550 nm light. Photons = 1.0 × 10⁻¹⁶ / 3.61 × 10⁻¹⁹ ≈ 277. The popular information says a CCD catches up to 90 of every 100 incoming photons, against about 1 for a photographic plate or the eye. At 90 per cent, about 249 electrons; at 1 per cent, about 3.
What mistakes do students make with these numbers?
Leaving λ in nanometres while h is in J s. Convert to metres, or use the 1240 eV nm shortcut consistently.
Mixing up work function and band gap. In the photoelectric effect electrons leave a metal surface; in a CCD or photodiode they stay inside the semiconductor and cross the gap.
Thinking more pixels means more light collected. Each pixel counts its own electrons; the count depends on the light that reaches that pixel.
The photoelectric effect is in Unit 17 and the photodiode in Unit 19 of the JEE (Main) 2026 syllabus; check the current year's version. Our post on photoelectric effect problems has more practice. If your photon counts keep coming out wrong, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
What can a student take from how they worked?
Short, focused sessions can produce big ideas. The basic design came from one brainstorm.
Build quickly. A working prototype within a week settled whether the idea held.
Watch for the use you did not plan. They wanted a memory and made a camera.
Next step: redo steps 2 to 4 for red light at 650 nm and check that silicon still responds.
